1. Field of the Invention
The present invention relates to improvements in a hydraulic control device for controlling a hydraulically operated continuously variable transmission of a belt-and-pulley type for an automotive vehicle.
2. Discussion of the Prior Art
There is known a belt-and-pulley type continuously variable transmission for a motor vehicle which includes a pair of variable-diameter pulleys provided on a first and a second rotating shaft, respectively, a transmission belt which connects the pair of pulleys to transmit power from one of the first and second shafts to the other, and a first and a second hydraulic cylinder for changing effective diameters of the respective variable-diameter pulleys. A hydraulic control device for controlling such a belt-and-pulley type continuously variable transmission is known, for example, according to laid-open Publication No. 52-98861 (published in 1977) of unexamined Japanese Patent Application, in which the tension of the transmission belt is controlled primarily by regulating a pressure of a working fluid which is applied to the second hydraulic cylinder (provided on the driven shaft), while the speed ratio of the transmission is controlled primarily by regulating a rate of flow of the fluid into or from the first hydraulic cylinder (provided on the drive shaft).
In the hydraulic control device indicated above, however, a single line pressure produced in relation to the actual speed ratio of the transmission is directly applied to the second hydraulic cylinder for controlling the belt tension, and is indirectly applied to the first hydraulic cylinder via a flow control valve for controlling the speed ratio of the transmission. According to this arrangement, the area of the pressure receiving surface of the first hydraulic cylinder should be larger than that of the second hydraulic cyinder, in order to feed the fluid into the first hydraulic cylinder and thereby cause the effective diameter of this first cylinder to be larger than that of the second cylinder, so that the speed ratio of the transmission may be varied over a sufficiently wide range. The belt-and-pulley continuously variable transmission of this type therefore tends to have relatively large external dimensions, and a comparatively amount of the fluid should be supplied to the first hydraulic cylinder, whereby the operating response of the transmission is not satisfactorily high. Further, the relatively large parts of the first hydraulic cylinder and the relatively large volume of this cylinder means an accordingly large moment of inertia of the first rotating shaft, which makes it difficult to rapidly increase the speed of the first shaft and the speed of the engine directly coupled thereto, particularly when the transmission is shifted down (to decrease the speed ratio to lower the speed of the second shaft). Further, the first shaft may be temporarily driven by the inertia of the vehicle, which deteriorates the drivability of the vehicle.
There is also known another belt-and-pulley type continuously variable transmission as disclosed in laid-open Publication No. 58-29424 (published in 1983) of examined Japanese Utility Model Application. This transmission includes a shift control valve (four way valve) for controlling the speed ratio of the transmission by feeding a working fluid from a pressure source into one of the two hydraulic cylinders while permitting the fluid to be discharged from the other hydraulic cylinder. The transmission further includes an electromagnetic pressure relief valve for adjusting the pressure of the fluid which is released from the shift control valve. In this type of belt-and-pulley type continuously variable transmission, a relatively high line pressure produced by a hydraulic pump is applied to one of the two hydraulic cylinders whose pressure becomes higher than that of the other cylinder while power is transmitted between the first and second shafts, i.e., to the hydraulic cylinder provided on the drive shaft. In the meantime, a relatively low line pressure prepared by the electromagetic pressure relief valve is applied to the other hydraulic cylinder (provided on the driven shaft). This arrangement eliminates the need of increasing the pressure receiving area of the first hydraulic cylinder as compared with that of the second hydraulic cylinder, and is therefore free of the drawback which arises from the comparatively large pressure receiving area of the first cylinder. In addition, the instant arrangement is advantageous in that the tension of the transmission belt can be suitably controlled by the relatively low line pressure produced by the electromagnetic pressure relief valve.
In the hydraulic control arrangement described just above, the relatively high line pressure produced by the hydraulic pump and supplied to the shift control valve is not regulated at all. Therefore, this line pressure must be set to a sufficiently high level, in order to adequately control the speed ratio of the continuously variable transmission under varying operating conditions. In other words, an extra load is exerted to the hydraulic pump under the normal operating condition, resulting in a corresponding amount of power loss of the pump, which leads to undesirable reduction in the fuel economy of the vehicle.